Arrangement for generating a 3:2 pull-down switch-off signal for a video compression encoder

ABSTRACT

Arrangement for generating a pull-down switch-off signal The invention relates to an arrangement ( 1 ) for generating a pulldown switch-off signal for a video compression encoder, which signal is determined by the arrangement ( 1 ) in dependence on a converted signal which is produced from an NTSC signal by means of an inverse 3:2 pull down conversion, wherein the circuit arrangement includes a M(ean) A(bsolute) D(istortion) (MAD) detector ( 2 ) and a circuit ( 3 ) for determining Hadamard coefficients, wherein the MAD detector ( 2 ) produces a MAD signal which indicates for each block of predefined size the difference between the picture contents of two consecutive frames, wherein the circuit ( 3 ) for determining the Hadamard coefficients delivers two coefficients in blocks per frame, from which coefficients a first coefficient indicates the sum of the differences of the pixels of adjacent scanning lines i and i+1 and a second coefficient indicates the sum of the differences of the pixels of scanning lines i and i+2, and wherein the pull-down switch-off signal is generated in dependence on the summed values of the MAD signal for all blocks of a frame and in dependence on the two Hadamard coefficients summed for all the blocks of a frame.

RELATED APPLICATIONS

This application claims the priority benefits of PCT InternationalApplication PCT/IB2004/052860, filed Dec. 20, 2004, which claimspriority to European Patent Application No. 04100052.2, filed Jan. 9,2004.

The invention relates to an arrangement for generating a pull-downswitch-off signal, which signal is determined for a video compressionencoder which may be, for example, an MPEG2 encoder. The arrangementthen produces this pull-down switch-off signal in dependence on aconverted signal which is produced from an NTSC signal by means of aninverse 3:2 pull-down conversion.

A what is called 3:2 pull-down conversion is applied to such NTSCsignals that have emerged from the scanning of a film which is scannedwith 24 frames per second. This scanning signal is then to be convertedinto an NTSC video signal with 60 fields per second. If each scannedframe were then to be used for generating two fields, only 48 fields persecond would evolve. Therefore, frames are alternately scanned threetimes in order to generate 3 equal fields. For the result this meansthat the frames of the film are scanned in a 3:2:3:2 cycle etc., so thatthe 24 frames become 60 fields of the video signal per second.

For example for DVD recorders, but also for recording such video signalson hard disks or for digital transmission of said video signals, it isdesired to subject such an NTSC video signal coming from a 3:2 pull-downconversion to a video compression, for example an MPEG2 compression.Since the data rate is always critical for such video compressions,there is a desire to avoid as much as possible that the same fields arescanned twice. Just this is possible in principle in an NTSC signal thatwas subjected to a 3:2 pull-down conversion, because fields that need tobe scanned and compressed only once are available double here. Thiscould lead to a reduction of about 20% of the video data for the videocompression, so that the bit rate could be increased accordingly.

From the state of the art solutions are known which therefore subjectthe NTSC signal, which has arisen from a 3:2 pull-down conversion of afilm scanning, to a what-is-called inverse 3:2 pull-down conversion. Thefields that have the same content, which have arisen from the doublescanning of the same frame, are then again rejected. A video streamevolves which contains frames whose fields have all arisen from thescanning of different pictures of the film from which the NTSC signalhas emerged.

The problem of such known arrangements consists of the fact that theapplication of the inverse 3:2 pull-down conversion to the NTSC signalthen leads to considerable errors in the representation, moreparticularly representation of motion when the NTSC signal, which issubjected to the inverse 3:2 pull-down conversion, has not or has nolonger emerged from a film scanning with 24 frames per second, but, whenit is a normal video signal, which contains 60 different fields,contains various motion phases per second. If the inverse 3:2 pull-downconversion is further applied to such a signal, fields are rejected thatactually have new picture content and content different from the otherfields. Particularly with motion there are distinct errors in thesignal.

Therefore, it is an object of the invention to provide an arrangementfor generating a pull-down switch-off signal which detects in asreliable and fast a manner when the NTSC signal applied to thearrangement, which NTSC signal was subjected to an inverse 3:2 pull-downconversion, was not generated or no longer generated by scanning a filmwith 24 frames per second and applying the 3:2 pull-down technique.

This object is achieved according to the invention by the featuresdefined in patent claim 1:

An arrangement for generating a pull-down switch-off signal for a videocompression encoder, which signal is determined by the arrangement independence on a converted signal which is produced from an NTSC signalby means of an inverse 3:2 pull-down conversion, wherein the circuitarrangement includes a Mean Absolute Distortion (MAD) detector and acircuit for determining Hadamard coefficients,

wherein the MAD detector produces a MAD signal which indicates for eachblock of predefined size the difference between the picture contents oftwo consecutive frames,

wherein the circuit for determining the Hadamard coefficients deliverstwo coefficients in blocks per frame, from which coefficients a firstcoefficient indicates the sum of the differences of the pixels ofadjacent scanning lines i and i+1 and a second coefficient indicates thesum of the differences of the pixels of scanning lines i and i+2,

and wherein the pull-down switch-off signal is generated in dependenceon the values of the MAD signal summed for all the blocks of a frame andin dependence on the two Hadamard coefficients summed for all the blocksof a frame.

The arrangement according to the invention is configured such that it isin a position to detect extremely fast and reliably from the signalgenerated from an NTSC signal by means of an inverse 3:2 pull-downconversion, when the NTSC signal has actually emerged from the scanningof 60 fields per second and no longer from the scanning of 24 frames persecond of a film and successive application of a 3:2 pull-downconversion.

For the reasons described above it is desirable for the arrangement todetect this transition in an extremely fast and reliable manner. Forthis purpose, on the one hand a what-is-called MAD detector as iscustomarily present in MPEG encoders is provided in the circuitarrangement. Such MAD detectors, where MAD stands for Mean AbsoluteDistortion, are generally used for estimating motion. Consecutive framesare then compared in blocks and it is then determined how much picturecontent per block has changed from one frame to the next.

The arrangement according to the invention further includes a circuitfor determining Hadamard coefficients. Two coefficients per block arethen generated for each frame. A first Hadamard coefficient sums inblocks the differences of the pixels of adjacent scanning lines i andi+1 within the block. For generating a second Hadamard coefficient, alsothe sum of the differences of pixels within the block is determined, butof pixels of the scanning lines i and i+2, thus every second scanningline. In this way the Hadamard coefficients represent in proportion toeach other a measure that expresses whether picture content of adjoiningscanning lines or of the adjacent-but-one scanning lines differs fromeach other. This may be considered a measure whether the frame hasarisen by scanning individual, different fields, or whether the framehas arisen from scanning a frame with a motion phase as this is thecase, for example, for film scanning. The calculation of the Hadamardcoefficients as such is known from “MPEG Video Compression Standard,Mitchell, Pennebaker, Fogg and LeGall, published by Chapman and Hall,1996.

Both the values of the MAD signal generated in blocks, and the first andsecond Hadamard coefficients generated in blocks are summed for I frame.

Within the context discussed above the arrangement can directly deducefrom these sums whether the signal subjected to an inverse 3:2 pull-downconversion is or is not the result of film scanning with 24 frames andsuccessive 3:2 conversion. This criterion can particularly be generatedbased on the Hadamard coefficients. The MAD signal additionally providesa kind of scene detection, because the MAD values rise considerably withchanging scenes. Similarly also holds true when the pull-down cycle wasdistorted during the generation of the NTSC signal, or when this signalwas later subjected to a cut, so that the 3:2 pull-down cycle in theNTSC signal is no longer available free of distortion. In all thesecases the arrangement according to the invention produces a switch-offsignal that can be used, for example, in an externally provided videocompression encoder for switching off the inverse 3:2 pull-downconversion. In this way the arrangement according to the invention isnot only suitable for recognizing a transition from an NTSC signalobtained from film scanning to a “normal” NTSC signal obtained fromvideo scanning with 60 fields per second, but it can also recognize thedetection of an erroneous 3:2 pull-down cycle. Furthermore, picturecontent strongly changing from one frame to the next can also be usedfor generating the pull-down switch-off signal. It is then alwaysguaranteed that with every distortion or with strongly changing picturecontent the inverse 3:2 pull-down conversion is switched off. This isadvantageous, because the inverse 3:2 pull-down conversion, if appliedwrongly, generates large evident distortions in the video signal.Therefore, it is suitable to switch off the 3:2 pull-down conversion incase of doubt.

According to one embodiment of the invention as claimed in claim 2, thepull-down switch-off signal is either generated if the MAD value of theindividual blocks summed per frame exceeds a predefined threshold, or ifthe quotient from the Hadamard coefficients generated per frame alsoexceeds a predefinable threshold within a predefinable number ofpull-down four-cycles of the converted signal. For this purpose, thefirst and second Hadamard coefficients which are generated in blocks, itis true, are summed for a respective frame. Subsequently, from the sumof the first Hadamard coefficient of a frame and the sum of the secondHadamard coefficient of a frame, the quotient is formed i.e. the sum ofthe first Hadamard coefficient is divided by the sum of the secondHadamard coefficient of the frame. If this value exceeds a predefinablethreshold during a predefinable number of pull-down four-cycles of theconverted signal, this indicates that the fields of each frame representdifferent phases of motion. In its turn this points out that the NTSCsignal, which was subjected to the inverse 3:2 pull-down conversion, hasnot arisen from film scanning, but from a video signal with 60 fieldsper second, which represent different phases of motion, or that the 3:2pull-down cycle was distorted by editing.

Further embodiments of the invention as claimed in claims 3 and 4 relateto a further refined evaluation of the quotient of the summed Hadamardcoefficients. Particularly the pull-down four-cycle may be consideredand, advantageously, the re-evaluation of the quotient of the Hadamardcoefficients may preferably be concentrated on certain predefinablepositions within such a pull-down four-cycle. More particularly thepositions 1, 2 or 3 within such a pull-down four-cycle are eminentlysuitable for recognizing the type of the NTSC signal or the kind ofscanning of which this is the result. The reason for this is that theHadamard coefficients of these frames change considerably, when the 3:2pull-down cycle was distorted by editing, or when the video signalsubjected to the inverse 3:2 pull-down technique has no longer emergedfrom scanning 24 frames per second, but from 60 fields, which representdifferent motion phases.

According to a further embodiment of the invention as claimed in claim 6the circuits for determining the MAD values as well as the circuits fordetermining the Hadamard coefficients may be provided in common for anMPEG encoder and for the arrangement according to the invention. This ispossible because such circuit elements are also present in MPEGencoders. These circuit elements can be used for the arrangementaccording to the invention, so that the additional expenditure for thearrangement according to the invention and for the generation of thepull-down switch-off signal can be kept very low.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 1 gives a diagrammatic representation of an inverse 3:2 pull-downconversion,

FIG. 2 shows a block diagram of the arrangement according to theinvention,

FIG. 3 gives a diagrammatic representation of a frame whose 2 fieldsrepresent the same motion phases and

FIG. 4 gives a diagrammatic representation of a frame comprising 2fields that represent different motion phases.

As has already been explained above, an NTSC video signal, which isavailable with a frequency of 60 fields per second, can be recovered asa “normal” video signal by scanning 60 fields per second. Such a signalis generated, for example, by electronic cameras. The NTSC signal,however, may also be recovered by scanning a film which is availablewith 24 frames per second. In order to generate from the 24 frames persecond not only 48 fields, but 60 fields, which an NTSC signal is tohave per second, this signal may be subjected to a what-is-called 3:2pull-down technique, in which individual fields occur several times.

For a video compression it is no use compressing the same fields severaltimes. Therefore, it is appropriate to recognize which fields weregenerated several times and to exclude these fields. For this purpose awhat-is-called inverse 3:2 pull-down conversion is known, which doesexactly this and which again produces 24 frames in accordance with thescanned film for the purpose of video compression.

If a video compression encoder is exactly in this mode, there is aproblem, however, that the video signal may include distortions, forexample, from cuts or other effects, or that the NTSC signal is changedover to a signal that has been formed from 60 fields with differentmotion phases. In either case this inverse 3:2 pull-down conversioncauses considerably distorted motion to be represented in the encodedpicture, so that this case is to be avoided.

In order to eliminate this problem, the arrangement according to theinvention is provided which produces a switch-off signal if there is adistortion in an NTSC signal which has been formed from the scanning of24 frames of a film and subsequent implementation of the known 3:2pull-down technique, so that the cycle of the 3:2 pull-down conversionis distorted or if the signal is changed over to a video signal with 60fields of different motion phases.

In order to achieve this object, the arrangement according to theinvention comprises a what-is-called Mean Absolute Distortion detector,which is generally known as MAD detector and which is used, for example,for motion detection. In the arrangement according to the invention thisdetector is used for generating hard cuts i.e. detecting stronglychanging picture content and generating the pull-down switch-off signal.The MAD detector produces a MAD signal which indicates, prior to blocksof a certain size within a frame, the difference of the picture contentof two successive frames. These MAD values generated block by block aresummed for 1 frame each time and the summed values of successive framesare compared with each other. If the difference exceeds a thresholdwhich can represent, for example, three times the mean value of the MADvalues of a predefinable number of previous frames, it indicates achange of scene or a hard cut. In that case the arrangement according tothe invention generates the pull-down switch-off signal, because it isalways suitable with such hard cuts to check the 3:2 pull-down cycle soas to avoid any picture distortions as a result of the inverse 3:2pull-down conversion of an input signal whose 3:2 pull-down cycle wasdistorted by editing, or with which a conversion from a film scanningsignal to a normal video signal with 60 fields per second has takenplace.

The arrangement according to the invention further includes a circuitfor determining Hadamard coefficients. Hadamard coefficients arecoefficients which are generated from frames block by block. A firstHadamard coefficient then takes into account the differences of thepixels of adjacent scanning lines of a frame and a second Hadamardcoefficient the sum of the differences of the pixels of scanning linesi, i+2, thus of every adjacent-but-one scanning line. These Hadamardcoefficients are then generated in blocks and successively summed for aframe. From the quotient of the first Hadamard coefficient divided bythe second Hadamard coefficient, which will further be explained below,it may further be concluded whether the signal subjected to the inverse3:2 pull-down conversion as before and undistorted by the 3:2 pull-downconversion has arisen from the scanning of a film with 24 frames persecond. FIG. 1 gives a diagrammatic representation of the arrangement 1according to the invention with a Mean Absolute Distortion detector 2and a circuit 3 for determining the Hadamard coefficients.

FIG. 1 shows that the arrangement 1 according to the invention issupplied with an NTSC signal on its input, which NTSC signal wassubjected to an inverse 3:2 pull-down conversion. The reason for this isthat the arrangement according to the invention is then to supply aswitch-off signal when an NTSC signal is subjected to an inverse 3:2pull-down conversion in a video compression encoder not belonging to thearrangement according to the invention and the criteria for thisconversion, however, are actually no longer satisfied. Thus as aninitial status it is always assumed that the NTSC signal is subjected toan inverse 3:2 pull-down conversion and that criteria are searched forthat point out that this conversion is to be switched off. At this verypoint the arrangement 1 according to the invention generates a pull-downswitch-off signal referred to as P in FIG. 1.

FIG. 2 shows in a diagrammatic representation in its first column framesof an NTSC signal which has arisen from a frame scanning of a filmavailable with 24 pictures per second. Such an NTSC signal would atfirst not be according to standard because it has 48 fields per secondand 24 frames per second. Therefore, a what-is-called 3:2 pull-downconversion is used for these 24 frames per second, which pull-downconversion generates a standardized 60 Hz NTSC signal from the signal.This 60 Hz NTSC signal is shown in the second column of therepresentation in FIG. 2.

Basically, this 3:2 pull-down NTSC signal could be used for a videocompression. The representation shown in FIG. 2, however, shows thatindividual fields of the frames of the first column show up varioustimes in the frames of the second column. For example, already the firstfield of the first frame of column 1 is used both for a field of theframe A of column 2 and of the frame B of the column 2. For a videocompression this means nothing more than that the same field is to besubjected to the (same) compression twice. This is inappropriate,because video compressions are always about obtaining a smallestpossible data rate. Therefore, a compression of the same fields is to beavoided.

For this reason a what-is-called inverse 3:2 pull-down conversionaccording to the state of the art is known, which conversion generatesfrom the 60 Hz NTSC signal as is represented in the second column ofFIG. 2 again a 48 Hz frame signal in which no fields occur twice.Furthermore, with the inverse pull-down conversion it is to be heededthat the fields of the original frames of the first column in therepresentation as per FIG. 2, as they have emerged from the filmscanning, are again correctly combined. The third column of therepresentation as per FIG. 2 shows the frames that have arisen from thisinverse 3:2 pull-down conversion. This is again a four-cycle.

The representation as per FIG. 2 also shows that certain fields, that isthe first field of frame B of the second column and the second field offrame C of the second column, are rejected because exactly these fieldshave arisen from double evaluation of the frames of the film scanning.The representation of FIG. 2 as a whole shows that as a result of theuse of the 3:2 pull-down conversion and the consecutive inverse 3:2pull-down conversion, again the right frames as they have originallyarisen from the film scanning are combined and that also the four-cycleagain arises. In this respect a video compression can take place whichcan work with an optimally low data rate, because no fields of the samecontent need to be compressed twice.

However, there is a problem if the pictures present with 60 Hz fieldfrequency of the second column of the representation of FIG. 2 haveeither no longer arisen from a frame scanning of a film available with24 frames per second, or when there is a distortion in this scanning,for example, as a result of cuts. If, for example, either the correctprocess of the four-cycle is no longer guaranteed, or a conversion hastaken place from a film scanning signal to a normal video signal with 60fields per second, the inverse 3:2 pull-down conversion leads to thefact that either the wrong fields are rejected, or that complete motionphases are rejected. In either case the video compression can certainlybe converted again to a complete compression of all the fields of the 60Hz signal in accordance with the second column in the representationshown in FIG. 2. This is exactly the object of the arrangement accordingto the invention.

By determining the MAD values and comparing the picture content of twoframes, the arrangement according to the invention still goes one stepfurther and always generates a switch-off signal when there is stronglyvarying picture content, which refers to a cut or distortion in thepicture. As early as that will the inverse 3:2 pull-down conversion beswitched off.

In addition, in accordance with the formulae

${{First}\mspace{14mu}{Hadamard}} = {\sum\limits_{0}^{13}\;{\sum\limits_{0}^{15}\;{{{{{in}(i)}(j)} - {{{in}\left( {i + 1} \right)}(j)}}}}}$${{Second}\mspace{14mu}{Hadamard}} = {\sum\limits_{0}^{13}\;{\sum\limits_{0}^{15}\;{{{{{in}(i)}(j)} - {{{in}\left( {i + 2} \right)}(j)}}}}}$

two Hadamard coefficients in the arrangement 3 of the representation ofFIG. 1 are generated. As shown by these two equations, for thirteenscanning lines I and for fifteen pixels j the differences of pixelvalues are formed block-by-block. The first formula shows the generationof the first Hadamard coefficient, which sums the differences betweenthe pixel values of scanning lines i and i+1. The second formula showsthe generation of the second Hadamard coefficient which generates thesedifferences for the pixel values of scanning lines i and i+2. TheseHadamard coefficients are first generated for each frame block-by-block.They are then summed individually for each frame i.e. a sum of the firstHadamard coefficient of a frame and a sum of the second Hadamardcoefficient of the same frame is generated. From these sums the quotientis determined in that the sum of the first Hadamard coefficient isdivided by the sum of the second Hadamard coefficient. This quotient isthen also used for generating the switch-off signal.

This is basically possible because the two Hadamard coefficients inprinciple indicate whether the picture differences of adjacent scanninglines or of adjacent-but-one scanning lines are larger. This can befurther explained with reference to the representations of FIGS. 3 and4. In FIG. 3 is shown in a diagrammatic form a frame that comprises 2fields which have emerged from scanning of the same picture, forexample, a film picture. In this case the first Hadamard coefficientwill rather be smaller than the second Hadamard coefficient, becausehere the differences of the picture values increase the wider apart thescanning lines are. FIG. 4 shows in a diagrammatic representation aframe which comprises two fields which represent different motionphases. In this case the first Hadamard coefficient will rather belarger than the second coefficient, because adjacent scanning lines ofthe frame have resulted from various fields of different motion phases.On the other hand, the respective adjacent-but-one scanning lines of theframe have emerged from a field of a certain picture phase, thus aremore likely to have slight differences of the picture values.

These connections are evaluated in the arrangement according to theinvention in so far that the quotient from the summed first and secondHadamard coefficients utilizes exactly this coherence discussed withreference to FIGS. 3 and 4. Therefore, this quotient may advantageouslybe used for detecting an undistorted NTSC signal resulting from filmscanning and be subjected to a 3:2 pull-down conversion. In anadvantageous manner certain positions of the signal subjected to thefour-cycle of the inverse 3:2 pull-down conversion in accordance withcolumn 3 of the representation of FIG. 2 can be used. If, for example,an NTSC signal resulting from 24 pictures of a film is not or is nolonger concerned, but the normal video signal with 60 fields ofdifferent motion phases is, the first Hadamard coefficient in frame 2 ofthe four-cycle will rise or fall. On the other hand, an extreme value ofthe quotient of the Hadamard coefficients of the frames 1 and 3 of thecycle points out that the signal that emerged from the scanning of thefilm was processed wrongly. Therefore, this may either be a distortionof the 3:2 pull-down conversion or a hard cut which was added to thescanned signal.

A special evaluation of the Hadamard coefficients in predefined positionwithin the pull-down four-cycle can thus also be used for improving thedetection and making it possible to generate the pull-down switch-offsignal in an optimally reliable manner.

1. An arrangement for generating a pull-down switch-off signal for avideo compression encoder, which signal is determined by the arrangementin dependence on a converted signal which is produced from an NTSCsignal by means of an inverse 3:2 pull-down conversion, wherein thecircuit arrangement includes a Mean Absolute Distortion detector and acircuit for determining Hadamard coefficients, wherein the MAD detectorproduces a MAD signal which indicates for each block of predefined sizethe difference between the picture contents of two consecutive frames,wherein the circuit for determining the Hadamard coefficients deliverstwo coefficients in blocks per frame, from which coefficients a firstcoefficient indicates the sum of the differences of the pixels ofadjacent scanning lines i and i+1 and a second coefficient indicates thesum of the differences of the pixels of scanning lines i and i+2, andwherein the pull-down switch-off signal is generated in dependence onthe values of the MAD signal summed for all the blocks of a frame and independence on the two Hadamard coefficients summed for all the blocks ofa frame.
 2. An arrangement as claimed in claim 1, characterized in thatthe pull-down switch-off signal signals a switching off when the MADvalue summed for each frame exceeds a predefined threshold, and/orsignals a switching off when the quotient from the first Hadamardcoefficient summed frame-by-frame and the second Hadamard coefficientsummed frame-by-frame at one or more predefinable positions within apredefinable number of pull-down four-cycles of the converted signalexceeds a predefinable threshold.
 3. An arrangement as claimed in claim2, characterized in that the pull-down switch-off signal signals aswitching off of the inverse 3:2 pull-down conversion when at least onepredefinable position within a predefinable number of pull-downfour-cycles of the converted signal the value of the quotients of theassigned Hadamard coefficients lies a predefinable value above or belowthe average of the summed quotients of the Hadamard coefficients of allthe positions of this pull-down four-cycle.
 4. An arrangement as claimedin claim 3, characterized in that the pull-down switch-off signalsignals a switching off of the inverse 3:2 pull-down conversion when atone of the positions one, two or three within three consecutive cyclesof the converted signal the value of the summed quotients of theassigned Hadamard coefficients lies about 10% above or below the averageof the quotients of the Hadamard coefficients of all the position ofthis pull-down four-cycle, wherein the position two within one cycle ofthe converted signal represents the position whose converted frame wasrecovered from two different frames of the unconverted signal.
 5. Anarrangement as claimed in claim 1, characterized in that the pull-downswitching signal signals a switching off of the inverse 3:2 pull-downconversion if the MAD signal summed frame-by-frame exceeds three timesthe average value from the MAD values of a predefinable number ofprevious frames.
 6. An arrangement as claimed in claim 1, characterizedin that the MAD detector and the circuit for determining the Hadamardcoefficients are provided in common for the arrangement and for an MPEGencoder for which the pull-down switch-off signal is provided.
 7. Anarrangement as claimed in claim 1, characterized in that the pull-downswitch-off signal is provided for an MPEG2 or MPEG4 encoder.
 8. Use ofthe arrangement as claimed claim 1 in a DVD recorder.
 9. A method forgenerating a signal, comprising: receiving an NTSC signal; processingthe NTSC signal by a Mean Absolute Distortion detector to produce a MADsignal which indicates for each block of predefined size the differencebetween the picture contents of two consecutive frames, generating, by acircuit for determining Hadamard coefficients, two coefficients inblocks per frame, from which coefficients a first coefficient indicatesthe sum of the differences of the pixels of adjacent scanning lines iand i+1 and a second coefficient indicates the sum of the differences ofthe pixels of scanning lines i and i+2; and generating a pull-downswitch-off signal based upon the values of the MAD signal summed for allthe blocks of a frame and the two Hadamard coefficients summed for allthe blocks of a frame.
 10. The method of claim 9, wherein the pull-downswitch-off signal: signals a switching off when the MAD value summed foreach frame exceeds a predefined threshold, and/or signals a switchingoff when the quotient from the first Hadamard coefficient summedframe-by-frame and the second Hadamard coefficient summed frame-by-frameat one or more predefinable positions within a predefinable number ofpull-down four-cycles of the converted signal exceeds a predefinablethreshold.
 11. The method of claim 10, wherein the pull-down switch-offsignal signals a switching off of an inverse 3:2 pull-down conversionwhen at least one predefinable position within a predefinable number ofpull-down four-cycles of the converted signal the value of the quotientsof the assigned Hadamard coefficients lies a predefinable value above orbelow the average of the summed quotients of the Hadamard coefficientsof all the positions of this pull-down four-cycle.
 12. The method ofclaim 11, wherein the pull-down switch-off signal signals a switchingoff of the inverse 3:2 pull-down conversion when at one of the positionsone, two or three within three consecutive cycles of the convertedsignal the value of the summed quotients of the assigned Hadamardcoefficients lies about 10% above or below the average of the quotientsof the Hadamard coefficients of all the position of this pull-downfour-cycle, wherein the position two within one cycle of the convertedsignal represents the position whose converted frame was recovered fromtwo different frames of the unconverted signal.
 13. The method of claim9, wherein the pull-down switching signal signals a switching off of theinverse 3:2 pull-down conversion if the MAD signal summed frame-by-frameexceeds three times the average value from the MAD values of apredefinable number of previous frames.
 14. The method of claim 9,wherein the MAD detector and the circuit for determining the Hadamardcoefficients are provided within an MPEG encoder for which the pull-downswitch-off signal is provided.
 15. The method of claim 14, wherein theMPEG encoder is an MPEG2 or MPEG4 encoder.
 16. The method of claim 9,wherein the method is performed by a DVD recorder.